The present invention is generally directed to optical elements and, more particularly, to apparatus for altering the focus distance of a lens assembly.
A major market for optical systems and devices is portable electronic equipment such as cellular telephones, pagers, portable computers and the like. Generally, it is desirable for the information displays used in devices of this type to be compact, with lower power requirements, inexpensive, and yet capable of providing a high quality image. As larger and more complex messages are being sent to remote units and more information is being displayed on portable computers, the ability of the display to present information to the user in a readable format is becoming more challenging. Prior art systems for displaying larger amounts of information typically comprise either a large display that is directly viewed, such as is used in a conventional portable laptop computer, or comprise a small image source with high magnification optics that create a virtual display. The major problem with a direct view system is that it greatly limits the minimum size of the portable electronics on which it is used. This is because the display must be large enough for an operator to read and understand the information being presented. High magnification virtual display units present substantial advantages over direct view systems for displaying large amounts of information using a compact optical package. Virtual image displays, however, generate a virtual image at an apparent distance that is fixed by the optical design and cannot be altered by the user merely shifting his or her viewing position. Since not all users of such devices will have the visual accommodation to focus at the identical image distance, but will have varying degrees of myopia, hyperopia and presbyopia, a virtual image formed at a point in space comfortable to one user may prove to be uncomfortable or even impossible for others to focus on clearly. Accordingly, it has been suggested that a focus mechanism that would permit the apparent location of the virtual image to be moved to various locations between the near point of the eye (25 cm) and infinity would permit individual users to adjust the apparent location of the virtual image for maximum comfort. One approach to providing such a focusing capability would be to provide means for moving the eye piece of a virtual display viewer relative to the image source either by physically moving the eye piece or the image source. This approach, however, presents substantial disadvantages in that very precise positional control must be maintained over the relative positions of the image source and the eye piece, otherwise misalignments can introduce unacceptable image degradations in all or portions of the image. The necessity of precise control over these optical elements therefore increases the complexity, manufacturing tolerances, and cost of such a focus mechanism.
What is needed is a focus mechanism that does not require the eye piece or the image source to be moved, so that the critical alignment between these optical elements can be fixed, thereby enabling a compact inexpensive optical package suitable for a portable electronic device to be realized.